Modern day radiation therapy (RT) of tumors can involve optimizing a target dose escalation for healthy tissue dose reduction and dose fractionation. It is known in the art that tumors can be eradicated if a sufficient dose is delivered to the tumor volume. However, complications may result from use of the necessary effective radiation dose, such as damage to healthy tissue which surrounds the tumor, or to other healthy body organs located close to the tumor. A goal of conformal radiation therapy is to confine the delivered radiation dose to only the tumor volume defined by the margins of the tumor, while minimizing the dose of radiation to surrounding healthy tissue or adjacent healthy organs.
In conventional radiation therapy with x-rays, intensity modulating radiation therapy (IMRT) offers an effective treatment for certain types of tumors and deep-seated lesions when a sufficient radiation dose is delivered. Cancer cells are often more sensitive to radiation damage than is surrounding healthy tissue due to inefficient repair. IMRT is delivered by external source of radiation from either a gamma emitter or linear accelerator.
The linear accelerator typically has a radiation beam source which is rotated about the patient and directs the radiation beam toward the tumor to be treated. The beam intensity of the radiation beam is predetermined and optimized for all azimuthal rotation angles. Multileaf collimators, which have multiple leaf, or finger, projections which can be moved individually into and out of the path of the radiation beam, can be programmed to follow the spatial contour of the tumor as seen by the radiation beam as it passes through the tumor, or the “beam's eye view” of the tumor during the rotation of the radiation beam source, which is mounted on a rotatable gantry of the linear accelerator. The multiple leaves of the multileaf collimator form an outline of the tumor shape as presented by the tumor volume in the direction of the path of travel of the radiation beam, and thus block the transmission of radiation to tissue disposed outside the tumor's spatial outline as presented to the radiation beam, dependent upon the beam's particular azimuthal orientation with respect to the tumor volume.
Tumors that are located deep within the body are generally not amenable to internal forms of treatment. The intrinsic nature of conventional radiation therapy with x-rays always includes damage to healthy tissue as it enters and exits the tumor volume and conformity is limited to the superposition of intersecting beams.
Another form of external beam radiotherapy is intensity modulated particle therapy (IMPT), which relies on the ballistic nature of particles to produce an inverse depth dose. Particle therapy typically utilizes an accelerator to generate high-energy protons to deposit dose in its path to a tumor before stopping at a precise depth known as its range. Particles heavier then protons, such as carbon ions, are additionally used to take advantage of higher linear energy transfer (LET) in causing more effective biological damage. Specifically, the charged particles damage the DNA within the cells, reducing the reproduction of the cell. The higher mass particles, such as carbon ions, can produce more DNA damage per unit of physical dose. This effect is characterized as relative biological effectiveness (RBE). Further, the larger mass associated with heavy ions, such as carbon, are characterized by reduced coulombic multiple scattering and range strangling. This can result in high spatial precision given the reduced lateral beam widening and sharper distal fall off in the tissue outside the tumor volume. The great advantage of particles, either protons or heavier ions, is the energy level stops, and thus, they do not produce an exit dose in the patient. This can result in reduction of side effects to surrounding tissue. All particles have a variety of energy levels that determines the depth of treatment of the tumor. Delivered to the tissue is a maximum deposition of energy just over the last few millimeters of the particles range called the Bragg peak. The Bragg peak is an inverse dose distribution level as shown in FIG. 1. The Bragg peak, indicated as reference number 1 in FIG. 1, demonstrates a low level of energy. As the particles are entering the human body, there is an increase in energy at a specific depth, which can be the region of interest. Thus, the high peak of energy should be within the tumor. This allows minimal damage to the surrounding tissue as compared to the actual tumor.
IMRT and IMPT can have positive and negative affects to the patient. IMPT takes advantage of both biological and physical effects. The first is for disease sites that favor the delivery of higher RBE radiation. Second, those treatments where the increased precision of particle therapy is used to reduce unwanted side effects by limiting the dose to normal tissue. In IMPT, the particle distributes high amounts of energy at a specific distance and then has minimal damage to the normal tissue. As opposed to IMRT which may deliver a high level of energy prior to entering into the tissue. Further, depending on the particle mass, IMPT can produce a narrower pencil beam, as opposed to conventional IMRT.
Currently, IMPT is delivered with passive double scattering and active scanning techniques. Double passive scattering is the most common technique that delivers a broad beam that must be adjusted with patient-specific hardware that shapes the beam to conform to the shape of the tumor. Passive double scattering, although still the most widely used technique, is being replaced with a process called active scanning or more commonly called pencil beam scanning (“PBS”) because of the correlation to the optimization algorithm for calculating dose in treatment planning systems (“TPS”). PBS was first introduced by T. Kanai et al. in 1980 and was developed at the Paul Scherer Institute in the mid 1990's. PBS delivers a much more precise beam and has superior 3D dose conformity as compared to passive double scattering.
In addition, there is the Spread Out Bragg Peaks, as indicated as reference number 3 as illustrated in FIG. 2. The Spread Out Bragg Peak (SOBP) can be used to demonstrate that particle therapy can distribute evenly throughout a tumor by superimposing multiple beams at varying energy. Currently, there are no known methods to deliver an entire SOBP dynamically. Further, there are fundamentally many inaccuracies when producing an SOBP sequentially for one energy per transverse scan.
Therefore, one of ordinary skill in the art can appreciate a need to precisely balance against the competing objective of destroying as much of the cancerous tissue as possible and in reducing exposure to healthy tissue. Thus, the objective is to deliver a dose sufficient to eradicate or dramatically reduce the tumor while minimizing the impact on surrounding normal tissue.
High-energy particles can be precisely formed into individual beams described as a pencil beam with spatial and angular dimensions. Charged particles, such as protons and carbon ions are characterized with inverse depth dose curves that have a specific range associated with particle kinetic energy. This unique dosimetric characteristic provides the 3rd dimension in producing a uniform dose volume with the ability to generate particles at specific energies corresponding to precise penetration depths (z-axis). This 3rd longitudinal dimension, when combined with the two transverse planes in the x-axis and the y-axis, requires scanning each pencil beam along the three axes (x-y-z). Each pencil beam is composed of an individual pristine Bragg peak that needs to be scanned in two orthogonal transverse (x-y) planes and one longitudinal z-axis. The pencil beam is physically repositioned for each transverse (x-y) position, while the longitudinal z-axis corresponds to the depth of the tumor and requires the generation of Bragg peaks of different energies, one for each depth. To create a uniform dose with depth, many pristine Bragg peaks are layered (stacked) one energy level per transverse scan cycle. A Spread Out Bragg Peak (SOBP), shown in FIG. 2, is typically generated after many transverse scan cycles and results in a uniform dose along the tumor depth. A dose distribution can be delivered conformal to a tumor volume of arbitrary shape using multiple pencil beams. The precision in conforming to the tumor volume is optimized by using pencil beams as small as possible. Therefore, a large number of pencil beams are required. A 3D volume can be decomposed into 3D pixels called voxels. For example, a 1 liter-cubic tumor volume would require over 1,000 x-y transverse positions and 62 energy steps (layers) for carbon ions, resulting in 68,000 individual voxels.
Therefore, one of ordinary skill in the art would appreciate a method of delivering ion radiation to a patient in fewer volumetric steps by reducing the scanning of the x-coordinate and y-coordinate, which causes latencies between scanning and delivering dose.